1. Field of the invention
The present invention relates to optical interference films. The optical interference film is generally used with a lamp to transmit light, the wave-length of which is within a specific range.
2. Description of the related art
Two different kinds of optical interference films are practically used. One of the two different kinds of optical interference films reflects infrared rays and transmits visible rays. The other optical interference film reflects visible rays and transmits infrared rays. A conventional optical interference film typically includes a dielectric layer and a metal layer each alternately accumulated one the other at a prescribed thickness. A thin layer of high refractive index substance, such as, e.g., titanium oxide (TiO.sub.2), zinc sulfide (ZnS), etc., is used as a dielectric layer. Silver is used as a metal layer. The above-described conventional interference film is disclosed in Japan Laid-open patent application (KOKAI) No. 53-146482. In such an optical interference film, light in a prescribed wave-length range is reflected and light in the remaining wave-length range is transmitted by the light interference phenomenon when the thickness of each refractive index layer is controlled at a prescribed value. The above-described conventional optical interference film has less heat resistance and less resistance to humidity. For example, if the conventional optical interference film is used at a temperature more than 200.degree. C. for an extended period, the optical characteristic thereof reduces. This is because silver in the metal layer is crystallized or oxidized by oxygen atoms of the dielectric layer. On the other hand, when zinc sulfide (ZnS) is used as the dielectric layer, the strength of the optical interference film is insufficient.